The 20-pS chloride channel of the human airway epithelium

The effect of mechanical deformation on the distribution of ions in fibroblasts

The extracellular and intracellular sodium, potassium and chloride concentrations were determined in fibroblast cells located in the rat calvarium. The ionic values were determined by fluorescence microscopy after incubation with the fluorescent probes, sodium-binding benzofuran isophthalate (SBFI), potassium-binding benzofuran isophthalate (PBFI) and 6-methoxy-N-(3-sulfopropyl) quinolinium (SPQ) (Dyes were supplied by CALBIOCHEM, Nottingham, England). After determination of the resting membrane potential, the calvaria were placed under tension by retraction of a micromanipulator. The fluorescence was measured again. A statistically significant difference was found in the calculated potassium ion concentration (Mann-Whitney; p < 0.05). This affected the resting cell membrane potential by an average of 5.2 mV. This effect was blocked by the addition of a potassium channel blocker, tetraethylammonium (TEA).

ATP-regulated chloride conductance in endoplasmic reticulum (ER)-enriched pig pancreas microsomes

The Cl- conductance of endoplasmic reticulum-enriched pancreatic microsomes was identified. Its regulation by nucleotides was investigated by measuring the rate of cation ionophore-induced microsome swelling in the presence of an inward Cl- gradient. The conductance was solubilized and reconstituted into liposomes. The Cl- conductance in intact microsomes was inhibited by stilbene (10(-4) M) and indanyloxyacetic acid (10(-5) M) derivatives. ATP increased Cl- conductance with half-maximal stimulation at 8 x 10(-6) M. Other trinucleotides (GTP, CTP and UTP) were without effect at 10(-4) M. The non-hydrolysable analogue of ATP, adenosine 5'-[beta gamma-methylene]triphosphate (AppCH2p) increased Cl- conductance with a potency similar to that of ATP. The same concentration of adenosine 5'-[gamma-thio]triphosphate (ATP gamma S) which is a substrate for kinases, had no effect. ATP stimulation of Cl- conductance was inhibited by stilbene derivatives. The data suggest the presence of at least one ATP-binding site, and show that the ATP does not need to be hydrolyzed and that its spatial conformation is important for activating the Cl- conductance. Solubilized microsomal proteins reconstituted into liposomes retained their stilbene-inhibited, ATP-stimulated Cl- conductance. A 167 kDa protein was detected by anti-CFTR antibodies in the intact microsomes, but not in the solubilized proteins. The 64 kDa protein (a component of a ubiquitous Cl- channel) was detected in the both intact and solubilized microsomes. These results suggest that this Cl- conductance is not a CFTR protein.

Independently gated multiple substates of an epithelial chloride-channel protein

We have purified a protein from Necturus maculosus gallbladder cells that forms chloride channels in an artificial membrane. The same protein apparently can form channels that are highly selective for chloride but can have conductances varying from 9 to about 150 pS. The high-conductance channels are blocked by the monoclonal antibody used to purify the protein, but this antibody has no effect on the 9-pS channels. The observation that gating of the low- and high-conductance states is independent and that the antibody affects only the latter has implications regarding the control of chloride conductance in cell membranes and the different types of channels described in those cells.

Halide permeation through three types of epithelial anion channels after reconstitution into giant liposomes

Anion-selective channels from apical membranes of cultured CFPAC-1 cells were isolated and incorporated into giant liposomes for patch clamp recording. Liposomes were formed from L-alpha-lecithin by a dehydration-hydration method. Ion channels were characterized using the excised inside-out patch clamp configuration. The most commonly observed anion channels were similar to those observed in native epithelial tissues. The linear 20 pS Cl- channel had the halide permeability sequence Cl- > I- > or = Br- > F-, and showed anomalous mole-fraction behavior in solutions containing different proportions of Cl- and F- ions. The autwardly rectifying Cl- channel had the halide permeability sequence I- > Br- > Cl- > F-, and also showed anomalous mole-fraction behavior, indicating that both these channels probably contain multi-ion pores. The third, voltage-dependent anion channel showed at least five different substrates, had a conductance of 390 pS in the main state, and showed two types of kinetics, fast (openings and closings < 1 ms), and slow (openings and closings > 1 s). The channel was seen more frequently after reconstitution into giant liposomes than in intact cells. It was not selective amongst the halides, and there was no deviation from a linear dependence of relative current on molar fractions, indicating relatively simple permeation through the pore. Differences in halide permeabilities suggest that different anion channels may be related to different membrane proteins. Comparison with the chloride channel proteins isolated biochemically from epithelial cell membranes is discussed.

A method for rapid exchange of solutions at membrane patches using a 10-microliters microcapsule

A rapid exchange (less than 2 ms) of the bath solution facing a membrane patch is accomplished by driving the tip of a pipette from the bath through a 100-microns oil layer into a small capsule filled with 10 microliters test solution. The microcapsule method can be applied to both excised patch configurations, inside-out and outside-out patches. On and off reactions of Ca(2+)-activated K+ channel activity have been recorded after changing the intracellular Ca2+ concentration using an inside-out patch. A blockade of these K+ channels by external tetraethylammonium ions is demonstrated with an outside-out patch. The blocking kinetics of delayed-rectifier K+ channels by a purified peptide toxin from snake venom, dendrotoxin, could be measured with our microcapsule method. Using tiny volumes of test solutions this method can be helpful in experiments involving scarce or expensive solutions.

GTP-binding proteins inhibit cAMP activation of chloride channels in cystic fibrosis airway epithelial cells

Cystic fibrosis (CF) is a genetic disease characterized, in part, by defective regulation of Cl- secretion by airway epithelial cells. In CF, cAMP does not activate Cl- channels in the apical membrane of airway epithelial cells. We report here whole-cell patch-clamp studies demonstrating that pertussis toxin, which uncouples heterotrimeric GTP-binding proteins (G proteins) from their receptors, and guanosine 5'-[beta-thio]diphosphate, which prevents G proteins from interacting with their effectors, increase Cl- currents and restore cAMP-activated Cl- currents in airway epithelial cells isolated from CF patients. In contrast, the G protein activators guanosine 5'-[gamma-thio]triphosphate and AlF4- reduce Cl- currents and inhibit cAMP from activating Cl- currents in normal airway epithelial cells. In CF cells treated with pertussis toxin or guanosine 5'-[beta-thio]diphosphate and in normal cells, cAMP activates a Cl- conductance that has properties similar to CF transmembrane-conductance regulator Cl- channels. We conclude that heterotrimeric G proteins inhibit cAMP-activated Cl- currents in airway epithelial cells and that modulation of the inhibitory G protein signaling pathway may have the therapeutic potential for improving cAMP-activated Cl- secretion in CF.

Ionic selectivity of volume-sensitive currents in human epithelial cells

The ion selectivity of swelling-activated Cl- currents has been investigated in three different human epithelial cell lines, two derived from the airway epithelium (9HTEo- and CFNPE9o-) and one from a colon carcinoma (T84). The relative permeability of volume-sensitive currents with respect to Cl- is: I- (1.19) greater than NO3- (1.07) approximately Br-(1.05) greater than Cl-(1.0) greater than F-(0.5) approximately HCO3-(0.48) greater than isethionate(0.28) greater than aspartate (0.14) approximately gluconate(0.13) approximately SO4(2-)(0.12). This type of ion selectivity is similar to that described for depolarization-activated outwardly rectifying Cl- channels found in epithelial cells.

Halide permeation through 10 pS and 20 pS anion channels in human airway epithelial cells

Halide permeability sequences were obtained from reversal potential measurements of single-channel currents through 10 pS and 20 pS anion channels in human airway epithelial cells. The sequences obtained were Cl- greater than I- greater than Br- greater than or equal to F- for the 10 pS channel and Cl- greater than I- greater than or equal to Br- greater than or equal to F- for the 20 pS channel. However, the permeability differences were not large, the greatest being 0.66 for the ratio of fluoride to chloride permeability in the 20 pS channel. Single-channel currents were also measured with solutions of constant halide concentration but varying ratios of chloride to fluoride ions. An anomalous mole fraction effect was observed for the 20 pS channel but not for the 10 pS channel, suggesting that the former is a multi-ion channel. Comparison of the halide permeability sequences of these two channels with those of whole-cell currents in other epithelial cells does not support their involvement in any of the known whole-cell epithelial currents.

Noise analysis and single-channel observations of 4 pS chloride channels in human airway epithelia

Apical membranes of human airway epithelial cells have significant chloride permeability, which is reduced in cystic fibrosis (CF), causing abnormal electrochemistry and impaired mucociliary clearance. At least four types of chloride channels have been identified in these cells, but their relative roles in total permeability and CF are unclear. Noise analysis was used to measure the conductance of chloride channels in human nasal epithelial cells. The data indicate that channels with a mean conductance of 4.5 pS carry most of the chloride current, and that the mean number of such channels per cell is approximately 4,000. Chloride channels in this conductance range were also seen in single-channel recordings.

Anion permeation in an apical membrane chloride channel of a secretory epithelial cell

Single channel currents though apical membrane Cl channels of the secretory epithelial cell line T84 were measured to determine the anionic selectivity and concentration dependence of permeation. The current-voltage relation was rectified with single channel conductance increasing at positive potentials. At 0 mV the single channel conductance was 41 +/- 2 pS. Permeability, determined from reversal potentials, was optimal for anions with diameters between 0.4 and 0.5 nm. Anions of larger diameter had low permeability, consistent with a minimum pore diameter of 0.55 nm. Permeability for anions of similar size was largest for those ions with a more symmetrical charge distribution. Both HCO3 and H2PO4 had lower permeability than the similar-sized symmetrical anions, NO3 and ClO4. The permeability sequence was SCN greater than I approximately NO3 approximately ClO4 greater than Br greater than Cl greater than PF6 greater than HCO3 approximately F much greater than H2PO4. Highly permeant anions had lower relative single channel conductance, consistent with longer times of residence in the channel for these ions. The conductance sequence for anion efflux was NO3 greater than SCN approximately ClO4 greater than Cl approximately I approximately Br greater than PF6 greater than F approximately HCO3 much greater than H2PO4. At high internal concentrations, anions with low permeability and conductance reduced Cl influx consistent with block of the pore. The dependence of current on Cl concentration indicated that Cl can also occupy the channel long enough to limit current flow. Interaction of Cl and SCN within the conduction pathway is supported by the presence of a minimum in the conductance vs. mole fraction relation. These results indicate that this 40-pS Cl channel behaves as a multi-ion pathway in which other permeant anions could alter Cl flow across the apical membrane.

The basic defect in cystic fibrosis

Recent evidence strongly suggests that the cystic fibrosis gene product (CFTR) is a Cl- channel. Its properties, however, differ from those of a 30-50 pS outwardly rectifying channel previously implicated as defective in cystic fibrosis. It is still uncertain whether the pleiotropic effects of the CF defect, such as increased airway Na+ absorption and mucus sulfation, are secondary to reduced Cl- conductance, or reflect additional functions of CFTR.

Characterization of a Ca(2+)-dependent anion channel from sheep tracheal epithelium incorporated into planar bilayers

1. Anion-selective channels from the apical membrane of respiratory epithelia are involved in the secretion of chloride into the airway lumen. In cystic fibrosis (CF) there is an abnormality of phosphorylation-regulated chloride transport in this tissue, whilst a calcium-dependent pathway appears to function normally. 2. Using incorporation of apical membrane vesicles into planar phospholipid bilayers, we have characterized the most commonly seen anion-selective channel from sheep tracheal epithelium. 3. In symmetrical 200 mM-NaCl solutions the channel showed rectification, with a chord conductance at negative voltages of 107 pS and at positive voltages of 67 pS. The channel characteristically demonstrated subconductance states at 1/3 and 3/4 of the fully open level. Selectivity for chloride over sodium was approximately 6:1. 4. The channel required a minimum of approximately 100 microM-calcium on the presumed cytoplasmic surface (cis) for opening events to be observed. Open probability (Po) of the fully open state was markedly voltage dependent, but little effect of voltage was seen on the 1/3 subconductance state. 5. The relative permeabilities of monovalent anions monitored under bi-ionic conditions gave the following sequence: NO3- greater than I- greater than Cl- = Br- much much greater than F-. The order of conductances in symmetrical solutions was Cl- = NO3- greater than Br- greater than I- much much greater than F-. 6. The chloride channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) produced a dose-related reduction in Po with a flickering block at 10-50 microM and complete block at higher concentrations. 7. ATP produced a dose-related reduction in Po with effects at 1 microM and complete closing at 1 mM. These effects were only seen with addition to the cis chamber. 8. The catalytic subunit of protein kinase A, either when incubated with vesicles prior to incorporation into bilayers, or when added directly to either chamber, produced no effect. 9. Channels with very similar properties were seen from transfected human tracheo-bronchial cells. 10. Recent whole-cell patch-clamp studies have suggested a distinct calcium-activated chloride current in secretory epithelia. The described channel has properties in common with this current and may be a candidate for its single-channel basis.

An analytical model of ionic movements in airway epithelial cells

A new mathematical model of ion movements in airway epithelia is presented, which allows predictions of ion fluxes, membrane potentials and ion concentrations. The model includes sodium and chloride channels in the apical membrane, a Na/K pump and a cotransport system for Cl- with stoichiometry Na+:K+:2Cl- in the basolateral membrane. Potassium channels in the basolateral membrane are used to regulate cell volume. Membrane potentials, ion fluxes and intracellular ion concentration are calculated as functions of apical ion permeabilities, the maximum pump current and the cotransport parameters. The major predictions of the model are: (1) Cl- concentration in the cell is determined entirely by the intracellular concentration of negatively charged impermeable ions and the osmotic conditions; (2) changes in intracellular Na+ and K+ concentrations are inversely related; (3) cotransport provides the major driving force for Cl- flux, increases intracellular Na+ concentration, decreases intracellular K+ concentration and hyperpolarizes the cell interior; (4) the maximum rate of the Na/K pump, by contrast, has little effect on Na+ or Cl- transepithelial fluxes and a much less pronounced effect on cell membrane polarization; (5) an increase in apical Na+ permeability causes an increase in intracellular Na+ concentration and a significant increase in Na+ flux; (6) an increase in apical Cl- permeability decreases intracellular Na+ concentration and Na+ flux; (7) assuming Na+ and Cl- permeabilities equal to those measured in human nasal epithelia, the model predicts that under short circuit conditions, Na+ absorption is much higher than Cl- secretion, in agreement with experimental measurements.

Cystic fibrosis, the CFTR, and rectifying Cl- channels

The human genetic disease cystic fibrosis is caused by a single defective gene on chromosome 7 that codes for a 1480 amino acid protein called the cystic fibrosis transmembrane conductance regulator (CFTR). The defect causes a profound reduction of Cl- permeability in several tissues, which in turn impairs salt absorption and fluid secretion. A 25-80 pS, rectifying Cl- channel has been targeted as the exclusive or primary channel affected in CF. However, we have found no evidence for significant activation or spontaneous activity of this channel in cell-attached patches of normal lymphoblasts or dog tracheal cells. However, in dog tracheal cells, we find lower conductance, linear Cl- channels that are spontaneously active in unstimulated cells and may show increased activity in stimulated cells. Attempts to correlate the expression of mRNA for the CFTR protein in various types of cells with the presence of the rectifying Cl- channel show a lack of correlation: i.e., depolarization-activated rectifying Cl- channesl have been found in excised, inside-out patches from all cell types that we have examined to date, but the CFTR mRNA has so far only been detected in a subset of epithelial cells.

Low-conductance chloride channel activated by cAMP in the epithelial cell line T84

We have studied the modulation and pharmacological properties of two anion channels in T84 cells by recording single channel and transepithelial currents. One channel had an outwardly rectifying current-voltage I/V curve, was rarely active in cell-attached patches, and was unaffected by cAMP. The other channel had lower conductance (8.7 pS at 37 degrees C) and a more ohmic I/V relationship. Exposure to cAMP increased the probability of observing low-conductance channel activity in cell-attached patches greater than 6-fold. Extracellular DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) or [IAA-94 (an indanyloxyacetic acid) inhibited the outward rectifier but did not affect the low-conductance channel or cAMP-stimulated transepithelial current. These results suggest the low-conductance Cl channel may contribute to apical membrane conductance during cAMP-stimulated secretion.

Separate Cl- conductances activated by cAMP and Ca2+ in Cl(-)-secreting epithelial cells

We studied the cAMP- and Ca2(+)-activated secretory Cl- conductances in the Cl(-)-secreting colonic epithelial cell line T84 using the whole-cell patch-clamp technique. Cl- and K+ currents were measured under voltage clamp. Forskolin or cAMP increased Cl- current 2-15 times with no change in K+ current. The current-voltage relation for cAMP-activated Cl- current was linear from -100 to +100 mV and showed no time-dependent changes in current during voltage pulses. Ca2+ ionophores or increased pipette Ca2+ increased both Cl- and K+ currents 2-30 times. The Ca2(+)-activated Cl- current was outwardly rectified, activated during depolarizing voltage pulses, and inactivated during hyperpolarizing voltage pulses. Addition of ionophore after forskolin further increased Cl- conductance 1.5-5 times, and the current took on the time-dependent characteristics of that stimulated by Ca2+. Thus, cAMP and Ca2+ activate Cl- conductances with different properties, implying that these second messengers activate different Cl- channels or that they induce different conductive and kinetic states in the same Cl- channel.